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| United States Patent |
5,001,517
|
|
Hosaka
, et al.
|
March 19, 1991
|
Developing apparatus for reverse-developing an electrostatic latent image
Abstract
A method and apparatus for development. A layer of developer which is a
mixture of insulative magnetic particles and toner particles is carried on
a surface of a developing sleeve which accommodates therein a magnet
roller. A latent image bearing member carrying a latent image to be
developed is so opposed to the developing sleeve that the latent image
bearing member to a portion of the magnet roller which is between the two
adjacent magnetic poles. The surfaces of the latent image bearing member
and the developer carrying member are maintained with a clearance which is
larger than the thickness of the developer layer. An alternating electric
field is formed in or across the clearance to alternatingly repeat two
steps, i.e., a toner transferring step wherein the toner particles are
transferred from the developer layer on the developer carrying sleeve to
the latent image bearing member, irrespective of whether it is the image
area or whether it is non-image area, and a back transfer step wherein
excessive toner particles are transferred back to the developer carrying
member, whereby a developer image is provided.
| Inventors:
|
Hosaka; Akihito (Yokohama, JP);
Kinoshita; Koichi (Shizuoka, JP)
|
| Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
530437 |
| Filed:
|
June 1, 1990 |
Foreign Application Priority Data
| May 16, 1984[JP] | 59-98424 |
| Jan 09, 1985[JP] | 60-1887 |
| Current U.S. Class: |
399/270; 399/274 |
| Intern'l Class: |
G03G 015/09 |
| Field of Search: |
355/251,265,268
118/657,658
430/100,122
|
References Cited
U.S. Patent Documents
| 3906121 | Sep., 1975 | Fraser et al. | 427/18.
|
| 4292387 | Sep., 1981 | Kanbe et al. | 430/102.
|
| 4363861 | Dec., 1982 | Nakamura | 430/97.
|
| 4468111 | Aug., 1984 | Yamagata et al. | 355/3.
|
| 4511239 | Apr., 1985 | Kanbe et al. | 355/3.
|
| 4525056 | Jun., 1985 | Masahiko et al. | 355/3.
|
| 4540645 | Sep., 1985 | Honda et al. | 430/122.
|
| 4557992 | Dec., 1985 | Haneda et al. | 355/14.
|
| 4579442 | Apr., 1986 | Minejima | 355/3.
|
| Foreign Patent Documents |
| 3102600 | Nov., 1981 | DE.
| |
| 3206815 | Mar., 1983 | DE.
| |
| 3427813 | Feb., 1985 | DE.
| |
| 3506311A1 | Sep., 1985 | DE.
| |
| 55-133069 | Oct., 1980 | JP | 355/3.
|
| 58-179880 | Oct., 1983 | JP | 355/14.
|
| 2095132A | Sep., 1982 | GB.
| |
Primary Examiner: Pendegrass; Joan H.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper & Scinto
Parent Case Text
This application is a continuation of application Ser. No. 244,605 filed
Sept. 6, 1988, now abandoned, which is a continuation of USSN 933,025
filed Nov. 20, 1986 now abandoned, which in turn is a divisional of
original USSN 731,039 filed May 6, 1985, now U.S. at. No. 4,653,427 issued
Mar. 31, 1987.
Claims
What is claimed is:
1. A developing apparatus for reverse-developing an electrostatic latent
image, comprising:
a developer carrying member which is movable along a closed path for
carrying a developer, said developer containing magnetic particles and
toner which is chargeable to a same polarity as that of an electrostatic
latent image;
magnetic field generating means disposed inside said developer carrying
member and having a plurality of magnetic field generating portions; and
means for forming an alternating electric field at a developing position
where the developer carrying member is opposed to a latent image bearing
member having the electrostatic latent image, said alternating electric
field forming means for applying to said developer carrying member an
alternating voltage superposed with a DC voltage which is between the
potentials at a light potential portion and a dark potential portion of
the electrostatic latent image.
2. An apparatus according to claim 1, further comprising a developer layer
thickness regulating member disposed upstream of the developing position
with respect to movement directly of said developer carrying member, said
regulating member forming the developer layer having a thickness smaller
than a gap between said developer carrying member and the latent image
bearing member at the developing position.
3. An apparatus according to claim 2, wherein the gap is 200-800 microns.
4. A developing apparatus for reverse-developing an electrostatic latent
image, comprising:
a developer carrying member which is movable along a closed path for
carrying a developer, said developer containing magnetic carrier and toner
which is chargeable to a negative polarity;
magnetic field generating means disposed inside said developer carrying
member and having a plurality of magnetic field generating portions;
means for forming an alternating electric field at a developing position
where the developer carrying member is opposed to a latent image bearing
member having an electrostatic latent image with negative polarity at both
light and dark potential portions, said alternating electric field forming
means for applying to said developer carrying member an alternating
voltage superposed with a negative DC voltage which is between the
potentials at the light potential portion and the dark potential portion.
5. An apparatus according to claim 4, further comprising a developer layer
thickness regulating member disposed upstream of the developing position
with respect to movement direction of said developer carrying member, said
regulating member forming the developer layer having a thickness smaller
than a gap between said developer carrying member and the latent image
bearing member at the developing position.
6. An apparatus according to claim 5, wherein the gap is 200-800 microns.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a developing method and a developing
apparatus of non-contact type for developing a latent image.
It is known, as disclosed in U.S. Pat. Nos. 4292387 and 4395476, for
example, that one component, insulative and magnetic toner or non-magnetic
toner is applied on a surface of a developer carrying member and is
conveyed thereon to a developing station, where the thin layer of the
toner is opposed with a clearance to a surface of a latent image bearing
member An alternating voltage is applied at this station so as to
transport the toner from the developer carrying member to the latent image
bearing member so as to develop the latent image, thus effecting the
developing action without contacting the thin toner layer with the latent
image bearing member.
However, the developing method or apparatus of this type involves the
following problem. When an attempt is made to positively charge with
certainty the toner particles on the developer carrying member surface to
the intended polarity which is required for the movement of the toner
particles in the developing station, the thickness of the toner layer on
the developer carrying member surface becomes thin so that the density of
the developed image is low, particularly in the case of a solid black part
of the image. This will be explained further. When a latent image
consisting of letters or characters is developed, the toner particles come
to the letter portions not only from the area of the toner layer exactly
opposed to the letter portions but also from the area in the neighborhood
thereof by the alternating electric field, whereby a developed image of a
sufficient density can be provided When, on the other hand, a solid black
image or a thick line image is developed, the amount of the thin layer
toner particles on the developer carrying member surface tends to be
insufficient, and the toner is concentrated at the edges of the latent
image with the result that the developed image is formed with an
insufficient amount of the toner particles.
When the magnetic toner particles each consisting of magnetic material and
resin, it is difficult to reproduce an image in a bright non-black color
since the magnetic toner particles contain the magnetic material, which is
usually black in color. Therefore, for a color reproduction, non-magnetic
toner mainly comprised of resin is used exclusively. However, the tendency
of the lack of the toner in the solid black image is observed more
remarkably when non-magnetic toner particles are used than when the
magnetic toner particles are used. For those reasons, the above described
problems are more significant when the color development is to be carried
out than when monochromatic development. Particularly, in the case of a
high quality development for a pictorial color reproduction, the above
described edge effects and the lack of density in the solid image are
significant problems.
Further, as one of non-contact type developing method, a proposal has been
made wherein a developer including insulating toner particles mixed with
conductive and magnetic carrier particles (hereinafter will be called
"conductive carrier") is applied on the surface of the developer carrying
member, the applied layer of the developer is opposed with a clearance to
the surface of the latent image bearing member with an alternating voltage
applied across the clearance so as to transit the toner particles from the
layer to the latent image bearing member to develop the same.
This type of developing method involves the following drawbacks. In the
developing action when the developer layer is opposed to the latent image
bearing member with the alternating voltage applied across the clearance,
the developer reciprocates across the clearance, resulting in the toner
particles deposited to the image part (the part to which the toner should
be deposited) of the latent image. It is required in order to obtain the
desirable development that only the toner particles reciprocate or
transit, but actually the conductive carrier particles also move to the
latent image bearing member. As a result, the carrier particles can
impinge on the toner particles already deposited on the image part of the
latent image, which causes the deposited toner particles to scatter
around. This disturbs the image to degrade the quality thereof
Additionally, when the conductive carrier particles reach the image part,
they neutralize the electric charge of the latent image, thus reducing the
image density. Furthermore, when the voltage is increased in an attempt to
broaden the area of the voltage application, a spark discharge can take
place across the clearance, which will destroy the latent image and
additionally which can damage the latent image bearing surface. This
occurs more easily when the resistance of the developer layer is low, so
that the tolerable range of the AC voltage is very narrow. Moreover, if
the conductive carrier particles are transferred to and deposited on the
latent image bearing member surface, they are not transferred to a
transfer material in the subsequent image transfer station, and therefore,
they reach the cleaning station by being conveyed on the latent image
bearing member. Then, the latent image bearing surface is "abraded" with
the conductive carrier particles, which can damage the latent image
bearing surface.
U.S. Pat. No. 4450220 proposes a non-contact developing method which uses
two component developer with insulative carrier particles. U.S. patent
application Ser. No. 632887 proposes a non-contact developing method with
the use of flat carrier particles. In those developing method, a
developing magnet pole is opposed to the latent image bearing member in
the developing station. Accordingly, the magnetic particles are formed
into a magnetic brush in the developing station so that the thickness of
the developer layer is not uniform there. This means that the distance or
clearance between the developer layer and the latent image bearing member
is not constant As a result, when an alternating voltage is applied across
the clearance, the strength of the electric field formed in the clearance
is not uniform, which may lead to the occurrence of undesirable discharge.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to provide a
developing method or a developing apparatus of a non-contact type which is
substantially free from all of the above described drawbacks, and which
exhibits less edge effect and can reproduce a solid black in a
satisfactory density. The apparatus and the method is applicable to a
pictorial color reproduction with a high quality of the image.
According to one aspect of the present invention, insulative and magnetic
particles are used and are mixed with insulative toner particles. The
mixture is applied on the developer carrying member as a developer layer.
The developer layer is opposed to the latent image bearing member with a
clearance. They are opposed in the position which is between magnetic
poles of magnetic field generating means. An alternating electric field is
formed in the developing station to transfer the toner particles. The
insulative magnetic particles do not transfer to the latent image bearing
member even when the electric voltage is applied, and no electric
discharge takes place. Therefore, a constantly stabilized quality of the
image can be provided.
In accordance with another aspect of the present invention the mixture is
applied on a developer carrying member as a developer layer. There is
provided inside the developer carrying member means for generating a
plurality of magnetic field portions having two adjacent magnetic field
portions of the same polarity. A scraper is placed in opposed relationship
to the developer carrying member between the two adjacent magnetic fields
of the same polarity. The developing layer is opposed to the latent image
bearing member with a clearance, and at that point, an alternating field
is formed to transfer the toner particles. A stirrer is disposed to stir
the developer in a manner which generates a frictional charge.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a developing apparatus according to an
embodiment of the present invention.
FIG. 2 is a somewhat schematic sectional view of a developing apparatus
according to another embodiment.
FIG. 3 illustrates a distribution of the magnetic field by a magnet roller
in the radial direction.
FIG. 4 illustrates a distribution of the magnetic field of the same in the
tangential direction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is shown a sectional view of a developing
apparatus according to a first embodiment of the present invention,
wherein a photosensitive drum, as the latent image bearing member, is
depicted by a reference numeral 1 and is effective to bear a latent image.
The developing apparatus comprises a developing sleeve 2 of non-magnetic
material, as the developer carrying member, and a fixed magnet roll 3 as a
magnetic field producing means. The developer which is a mixture of
ferrite carrier particles (magnetic particles) and toner particles is
designated by a reference numeral 4. The developer mixture is supplied to
the developing sleeve 2. The developing apparatus further includes a DC
electric power source 5 for supplying a DC voltage to the developing
sleeve 2, an AC power source 6 for supplying an AC voltage to the
developing sleeve 2, a scraper blade 7, a hopper 8 for accommodating the
toner particles, a toner supplying roller 9 and a stirring member 10.
The developer used in this embodiment was constituted by 75 g of ferrite
particles having the average particle size of 25 microns as the developer
particles and 25 g of the toner particles having the average particle size
of 15 microns which were positively chargeable. They were mixed together
The magnet roll 3 had the surface magnetic field strength of 1000 Gauss.
With this magnetic field, the above described developer was deposited on
the developing sleeve surface. The thickness of the layer of the developer
containing the toner and the magnetic particles was controlled by a doctor
blade 11 so as to provide a clearance of 100 microns in the position where
the developer layer is closest to the latent image bearing member surface.
The rotational directions of the latent image bearing member 1 and the
developing sleeve 2 are as shown by the arrows in FIG. 1 so as to provide
a higher relative speed therebetween. This is effective to increase the
image density. However, the latent image bearing member 1 and the
developing sleeve 2 may be rotated in such directions that they move in
the same direction in the developing station where they are opposed, or
that the relative speed therebetween is zero.
The back electrode of the latent image bearing member is grounded An
alternating voltage of 1 KHz having an effective voltage of 0.7 KV and a
DC voltage of -100 V were applied to the developing sleeve 2. The
clearance between the developing sleeve 2 and the latent image bearing
member 1 was 300 microns. With those structures, a latent image having a
dark potential of -600 V and the light (background) potential of -50 V was
developed. It has been confirmed that the dark part of the image is
visualized in a sufficient image density, and in the light part, no toner
is deposited there. The visualized image was transferred onto paper, film
and another toner image bearing member, thus providing the transferred
image. It is a possible alternative that the electrostatic latent image is
formed on electrostatic recording paper, and then it is fixed after
development.
In the embodiment described above, it has been confirmed that the toner
particles repeatedly reciprocate across the clearance between the latent
image bearing member 1 and the developing sleeve 2 when the electric
voltage is applied to form the corresponding electric field, however, the
magnetic particles do not move to the latent image bearing member. More
particularly, the magnetic particles can retain electrostatic charge of
the polarity opposite that of the toner particles, and therefore are
attached to the surface of the sleeve 2 by an image force of the
electrostatic charge on the sleeve side. Additionally, the magnetic
particles are retained by the magnetic retaining force applied thereto by
the fixed magnet roller existing behind the developing sleeve 2. For those
reasons, the magnetic particles are not transferred. Thus, the magnetic
particles do not disturb the image, and a high quality of the images is
provided.
When non-magnetic material having low resistance, for example, aluminum
powder, was mixed into the developer of this embodiment, that material
moved around at random back and forth and leftwardly and rightwardly
without the directional nature as in the toner reciprocation. Therefore,
it has been confirmed that it is preferable not to mix the low resistance
material, and that it is preferable for the magnetic particles to be
insulative. In the foregoing example, ferrite was used as for the magnetic
particles. The resistance of the ferrite was 10.sup.13 ohm.cm measured
when it is sandwiched without pressure between electrodes under the
electric field of 1000 V/cm.sup.2. It has been empirically confirmed that
the volume resistivity of the magnetic particles of the insulative nature
is preferably not less than 10.sup.12 ohm.cm, more preferbly not less than
10.sup.13 ohm cm. The average particle size of the ferrite is preferably
not more than 17 microns, because, if it is less, the magnetic force
received by the magnetic particles is weak enough to cause the transfer.
As for the insulative magnetic particles, besides the above described
ferrite, any material can be used, such as magnetic oxide powder of
electrically insulative nature and magnetic particles coated with
insulative resin, for example, the magnetic particles obtained by iron
particles having the average particle size of 25 microns coated with
acrylic resin having the thickness of approximately 5 microns. When
magnetic particles composed of a resin in which magnetic powder is
dispersed is used, the magnetic force received by the particle is reduced,
corresponding to the volume of the resin part, with the result that the
tendency is increased toward transferring to the surface of the latent
image bearing member. To obviate this, the particle size has to be larger
as compared with the case where only the ferrite is used. It follows that
when a smaller particle size is used, it is preferable that the entirety
of the magnetic particle or the entirety of the inside of the magnetic
particle is constituted by magnetic material. As for the shape of the
magnetic particle, a sphere is preferable because it is difficult that an
electric field concentration occurs and that the discharge occurs.
FIG. 2 is a sectional view of a developing apparatus according to another
embodiment of the present invention, wherein an electrostatic latent image
bearing member, designated by a reference numeral 1, includes a back
electrode 1a and an electrostatic latent image bearing layer 1b which may
be an insulating layer or an electrophotographic photosensitive layer. In
this embodiment it is shown as a photosensitive drum 1. The developing
apparatus comprises a developer carrying member 2 in the form of an
electrically conductive sleeve 2 of non-magnetic material rotatable in the
direction shown by an arrow A, magnetic field generating means 3 fixed
within the sleeve 2, in the form of a magnet roller having four magnetic
poles in this embodiment. Developer 4 includes magnetic particles M
containing a resin and magnetic powder therein and non-magnetic particles
(toner particles) T, mixed thereto, mainly composed of a resin and having
an average particle size which is smaller than that of the magnetic
particles. Between the sleeve 2 and the back electrode la of the
photosensitive drum 1 which is rotatable in the direction indicated by an
arrow B, a developing bias voltage is applied by a DC power source 5 and
an AC power source 6. The developing apparatus further comprises a
resilient member 12, a toner supplying roller 9 and a developer layer
regulating member 11 which is a doctor blade in this embodiment. The same
reference numerals as in FIG. 1 embodiment are assigned to the elements
having the corresponding functions.
The sleeve 2 is disposed, with a clearance of 200-800 microns, preferably
300-600 microns, opposed to the photosensitive drum 1 bearing an
electrostatic latent image to be developed on its surface and rotating in
the direction of the arrow B. The clearance can be formed and maintained
by known mechanism, for example, by spacer rolls concentrically mounted to
the opposite longitudinal ends of the sleeve 2 and kept contacted to the
surface of the photosensitive drum 1.
The toner supplying roller 9 has a plurality of recesses on the surface
thereof and rotates slowly by a driving gear (not shown) of the sleeve 2
meshed with a driving gear (not shown) of the photosensitive drum 1. With
the slow rotation of the toner supplying roller 9, the non-magnetic
particles (toner particles) T in the hopper 8 gradually fall into the
lower developer chamber in cooperation with the resilient member 12, thus
supplying the toner T thereto.
When the toner particles T are supplied to the developing chamber, they are
mixed with the magnetic particles M (the particles composed of the resin
containing the magnetic powder) existing in the neighborhood of the
surface of the sleeve 2 accommodating therein a magnet roller 3. When the
sleeve 2 rotates in the direction shown by the arrow A, the developer 4 in
the neighborhood of the sleeve surface moves as shown by an arrow C,
whereupon the toner particles supplied as described above are gradually
mixed into the developer 4.
The developer 4 thus mixed is formed into and applied on the sleeve surface
as a developer layer of a proper thickness, for example 100-600 microns,
preferably 150-500 microns, by the doctor blade 11. The doctor blade 11 is
opposed to the surface of the sleeve 2 at a position between the magnetic
pole N1 and S2 of the magnet roller 3 with a clearance, to the surface of
the sleeve, of approximately 100-550 microns, preferably 150-450 microns.
The doctor blade is fixed at such a position and made of nonmagnetic
material. The doctor blade is effective to regulate the thickness of the
developer layer applied on the surface of the sleeve 2. The thickness of
the developer layer is smaller than the clearance formed between the
photosensitive drum surface and the sleeve surface in the developing
station, so that the surface of the developer layer is out of contact with
the surface of the photosensitive drum 1 when not operated.
The non-magnetic particles (toner particles) T in the applied developer 4
have been triboelectrically charged by the friction with the magnetic
particles M and/or the friction with the surface of the sleeve 2. With
this charged state, the non-magnetic particles are attached by the
electrostatic force to the sleeve surface rotating in the direction
indicated by the arrow A and are attached to the magnetic particles by the
electrostatic force. Therefore, the non-magnetic particles are carried on
the surface of the sleeve 2 together with the magnetic particles and
conveyed to the developing zone by the rotation of the sleeve 2.
The developing apparatus is so opposed, in the developing zone to the
photosensitive drum 1 that the photosensitive drum 1 is opposed to the
position of the magnetic roller 3 between the magnetic pole Nl and the
magnetic pole S1. Therefore, the developer on the sleeve surface is not
formed into an upstanding magnetic brush, thus maintaining a constant and
uniform thickness of the layer. So, in order to maintain the developer
layer out of contact with the surface of the photosensitive drum 1, it is
not necessary to space the sleeve surface from the photosensitive drum
surface by a long distance, such as more than approximately 1 mm.
Accordingly, it is possible to dispose the sleeve 2 more closely to the
drum surface so that a clear and sharp images can be provided due to the
effect cf the developing electrode which works well when the clearance is
small. It has been confirmed that when the clearance is more than
approximately 1 mm, unsharp images result.
During the developing action, the alternating voltage is applied between
the sleeve 2 and the back electrode 1a of the photosensitive drum 1 so as
to form an alternating electric field in the developing zone. The
alternating voltage is formed with the DC voltage provided by the DC
source 5 and the AC voltage provided by the AC source, superposed thereto.
It is a possible alternative that only the AC voltage provided by the AC
source is used for the bias. The C voltage is not necessarily in the form
of a sine wave, but may be a rectangular or triangular wave. The AC
voltage is preferably 200 V-4 KV of the peak-to-peak voltage and 100-4 KHz
of the frequency.
EXAMPLE 1
With the developing apparatus described in conjunction with FIG. 2, the
development was actually carried out for an electrostatic latent image
having a dark area potential Vd of +600 V and a background area potential
Vl of 0 V. The developing bias voltage applied was obtained by superposing
a DC voltage of +150 V to an AC voltage having the frequency of 1.6 KHz
and the peak-to-peak voltage of 1800 Vpp. The non-magnetic particles was
toner particles having the weighted average particle size of approximately
8 microns containing as a main component a thermoplastic resin
(polystyrene). The non-magnetic particles were negatively chargeable with
respect to the magnetic particles. If positively chargeable toner
particles are used, a reverse development can be carried out, provided
that the DC voltage is selected suitably. The magnetic particles were
obtained by kneading resin material containing as a main component
styrene-acrylate-aminoacrylate copolymer and 75 wt. % of magnetic powder
of magnetite (Fe.sub.3 O4), and then pulverizing it into particles of the
weighted average particle size of 50 microns. Better images were provided,
when not more than 1 wt. % of silica particles were mixed into the above
described two component developer, the silica particles having a position
in the series of electrostatic charge between the positions of the two
particles of the developer.
When the above described bias voltage is applied, and the potential of the
sleeve 2 exceeds the threshold in the negative voltage phase, the
non-magnetic particles negatively charged are moved across the clearance
from the developer layer on the sleeve 2 to the surface of the
photosensitive drum 1 at least in the position where the drum 1 and the
sleeve 2 are most close, irrespective of whether it is the image area or
the non-image area (the background of the image). However, in the phase of
the opposite polarity, at least the excessive non-magnetic particles move
back to the sleeve 2. These steps are repeated a plurality of times, and
then the movement fade out with the reduction of the alternative electric
field together with the increase of the clearance between the drum 1 and
the sleeve 2, thus terminating the developing action. In order to reduce
the alternating electric field, the voltage applied may be decreased.
What is important here is that the magnetic particles are not transferred
to the photosensitive drum 1 from the developer layer on the sleeve 2. If
the transfer occurs, the magnetic particles in the developing apparatus
gradually decreases in the amount, resulting in that the ratio of the
number of magnetic particles to the number of non-magnetic particles
becomes excessively out of balance. If this ratio (toner
particles/magnetic particles) is excessively offset, the background fog
can result. It is, therefore, important to confine or retain the magnetic
particles on the sleeve surface by the magnetic force.
What is also important is that the photosensitive drum 1 and the sleeve 2
are not too distant in order to avoid unsharp images. If the drum 1 is
opposed to a magnetic pole in the developing zone, an upstanding magnetic
brush is formed, and therefore, it is difficult to reduce the distance
between the drum 1 and the sleeve 2, and additionally it is easy for the
undesirable discharge to take place.
In consideration of the above, it is significant that the photosensitive
drum 1 is opposed in the developing zone to the position of the magnet
roller 3 between the magnetic poles (N1 and S1 in this embodiment).
FIG. 3 illustrates a distribution of the magnetic field provided by the
magnetic roll 3. This Figure shows the distribution of radial component of
the magnetic field (the strength of the magnetic pole) which is generally
used to express the strength of the magnetic field on the sleeve surface.
In this Figure, the position of zero degree corresponds to the line, which
is horizontal in this embodiment, connecting the center of the drum 1 and
that of the sleeve 2. The radial component is the component of the
magnetic field extending perpendicularly to the surface of the sleeve 2,
and FIG. 3 shows the distribution thereof all around the sleeve surface.
It will be noted that the magnetic field is zero Gauss at a point between
the magnetic pole N1 and the magnetic pole S1. The experiments carried out
by the inventors showed that the magnetic particles were magnetically
retained on the sleeve 2 even at the position between the magnetic poles
and did not transfer to the photosensitive drum. This would not be readily
understood from FIG. 3. However, it would be understood readily in
conjunction with FIG. 4 which shows the distribution of the tangential
component of the magnetic field on the sleeve surface, using the same
coordinate axes. In this embodiment, the tangential component was 600
Gauss between the magnetic poles N1 and S1. It has been confirmed by many
experiments that a good quality of images is obtained if the tangential
component is not less than 200 Gauss, more preferably not less than 300
Gauss. This is because the magnetic particles are hardly deposited onto
the drum 1 when the magnetic field is not less than 200 Gauss. If it is
less than that, the magnetic particles tend to deposit onto the drum
surface, and this requires that the particle size of the magnetic
particles is increased, which results in a decreased concentration of the
toner/magnetic particles whereby the concentration control is difficult.
Additionally, the developer layer becomes thicker which leads to the
increased clearance between the drum 1 and the sleeve 2 so that unsharp
images result.
By opposing the position between the magnetic poles to the photosensitive
drum, the magnetic particles are effectively and substantially prevented
from transferring to the surface of the drum 1, and also the necessity of
increasing the clearance between the drum 1 and the sleeve 2 can be
eliminated, the increase being needed to avoid the influence of the
upstanding magnetic brush and necessarily resulting in unsharp image.
Accordingly, the above described conditions are important in this
embodiment.
Table 1 indicates the satisfactory conditions of the magnetic particles
composed of a resin containing magnetic powder to stably provide a good
quality without foggy background.
TABLE 1
______________________________________
Magnetic powder
Average content (wt. %)
particle size (.mu.m)
70 60 50 40 30
______________________________________
30 F N N N N
40 G F N N N
50 G F F N N
60 G G F N N
70 G G F N N
80 G G F F N
90 F F F F N
100 F F F F N
______________________________________
G: Good
F: Fair
N: Not Practical
In this Table "N" means not practical, "F" practically usable, and "G" more
preferable. When the weighted average particle size is less than 30
microns, the magnetic particles transfer tot he photosensitive drum 1,
which is not practically usable. In the range of the weighted average
particle size of 80-100 microns, it is practically usable if the magnetic
powder content is not less than 40 wt. %. Exceeding 100 microns, the
preferable content of the non-magnetic particles in the developer
approaches 10 wt. %, so that the mixture ratio becomes not so different
from that of conventional two component developer, which requires strict
control of the mixture ratio. It follows that the weighted average
particle size of the magnetic particles formed by the resin containing
therein the magnetic powder is preferably not less tan 30 microns, more
preferably not less than 40 microns but not more than 100 microns, further
preferably not less than 40 microns but not more than 80 microns. And, the
magnetic powder content of the magnetic particle is preferably not less
than 40 wt. %. The magnetic particle entirely composed of magnetic
material is usable. The magnetic particle formed by a core of magnetic
material and a coating therearound of a resin is usable. This is
advantageous since it can be easily formed into a spherical shape and can
be uniformly charged triboelectrically. A charge controlling agent which
may be a pigment or dye may be mixed into the resin constituting the
magnetic particle so as to ensure that the non-magnetic particles (toner
particles) are charged in the intended polarity and to the intended amount
of charge, thus providing a high quality of images.
The developed images with high image density and without a foggy background
could be obtained in the broad range of the mixture ratio of the toner
particles and magnetic particles, 15 wt. % to 45 wt. %. Because of this
broadness, the control of the toner concentration is easier, which is of
course advantageous. If the mixture ratio is less than 15 wt. %, the
density of the developed image is low, while if it is more than 45 wt. %,
the background fog results.
In the description of this embodiment of FIG. 2, the sleeve 2 has been
rotated in the direction of the arrow A. However, good images were also
obtained when it is rotated in the opposite direction. To rotate the
sleeve in the opposite direction rather than the direction of the arrow A
was found to be effective to increase the image density in a high speed
developing operation.
Regarding the magnetic retention or confinement of the magnetic particles,
the relation will be described between the strength of the tangential
component of the magnetic field on the sleeve surface between the magnetic
poles in the developing zone and the radial component thereof on the
sleeve surface at the position of the two magnetic poles (N1 and S1). A
stronger radial component of the magnetic field on the sleeve surface at
the positions of the magnetic poles, does not necessarily lead a stronger
tangential component of the magnetic field at a position between the
magnetic poles. If the two magnetic poles are too distant, the tangential
component on the sleeve surface at a position between the magnetic poles
is reduced, too. On the contrary, if the two magnetic poles are too close,
the area between the magnetic poles is narrow, resulting in a narrow
proper developing zone, and in addition, the strength of the tangential
component of the magnetic field of the sleeve surface is not increased
much. In consideration of those, the angle .theta. formed between a line
connecting the center of the magnet roller 3 (which is the center of the
sleeve 2) and one of the magnetic pole (Nl) and a line connecting the
center of the magnet roller 3 and the other magnetic pole (S1), is
preferably satisfies 45 degrees .ltoreq..theta..ltoreq.135 degrees.
In order to stably obtain a high quality of images, the weighted average
particle size Dt of the non-magnetic particles (toner) preferably
satisfies
Dt.ltoreq.Dc.ltoreq.15Dt
where Dc is the weighted average particle size of the magnetic particles.
If the particle size of the non-magnetic particles is too large as compared
with the particle size of the magnetic particles, the non magnetic
particles is insufficiently charged triboelectrically. If it is too small
on the contrary, a poor quality of images results.
Additional examples will be described together with a comparison example.
EXAMPLE 2
The clearance between the photosensitive drum 1 and the sleeve 2 was
maintained 500 microns, and the thickness of the developer layer was
regulated by the doctor blade 11 such that the thickness thereof was 400
microns in the zone where the developer layer was most closely opposed to
the photosensitive drum. As for the developer, non-magnetic particles
having the average particle size of 8 microns and magnetic particles are
mixed The non-magnetic particle concentration was 30 wt. %. The magnetic
powder content in the magnetic particle was 70 wt. %, and the average
particle size thereof was 50 microns The magnetic poles were disposed as
shown in FIG. 2 so that a position between the magnetic poles was opposed
to the drum 1. The magnetic poles N1 and S1 provided the magnetic field
having the radial component of 700 Gauss. The strength of the tangential
component between the magnetic poles was 610 Gauss.
Under the above conditions, development was carried out for an
electrostatic latent image having a dark potential Vd of -600 V and the
background potential Vl of 0 V. The non-magnetic particles (toner
particles) were positively chargeable. The developing bias used was
provided by superposing a DC voltage of -150 V with an AC voltage having
the peak-to-peak voltage Vpp of 1800 V and the frequency of 1.6 KHz. It
was recognized that only the non-magnetic particles transferred to the
image area (Vd), none of the magnetic particles and non-magnetic particles
were deposited on the non-image area, so that a good quality of image was
provided without a foggy background.
COMPARISON EXAMPLE
The developing operation was carried out under the same conditions as
Example 2 with the exception that the magnetic particle contained 30 wt. %
of the magnetic powder and had the average particle size of 40 microns.
The magnetic particles found to be deposited on the non-image area so that
a good image could not be obtained.
EXAMPLE 3
The developing operation was carried out under the conditions similar to
Example 2 so as to effect a reverse development wherein the light area of
the image, that is, the background area was visualized. In this Example,
the dark potential of the latent image was -600 V, and the light potential
Vl was -50 V. The non-magnetic particles used were negatively chargeable.
The developing bias applied was provided by superposing a DC voltage of,
-450 V with an AC voltage having the peak-to-peak voltage Vpp of 1800 V
and the frequency of 1.6 KHz. It was recognized that only the non-magnetic
particles were transferred to the light part of the drum which was the
image part in this Example, and that none of the magnetic particles and
non-magnetic particles was deposited to the dark part which was the
non-image area in this Example Thus, a good reverse development was
carried out.
Also, when the latent image as of positive polarity good images were
obtained as in the case described above by employing non-magnetic
particles positively chargeable and employing the DC voltage of +450 V.
As described in the foregoing, according to this embodiment of the present
invention, the surface of the developer carrying member behind which
magnetic field producing means is provided, carries a layer of a developer
including the magnetic particles containing not less than 40 wt. %
magnetic powder and including the non-magnetic particles mixed thereto and
mainly consisting of the resin. In the developing zone, the latent image
bearing member is opposed to a position between the magnetic poles of the
magnetic field producing means behind the developer carrying member. The
strength of the tangential component of the magnetic field on the
developer carrying surface is not less than 200 Gauss, preferably not less
than 300 Gauss. The clearance between the surfaces of the latent image
bearing member and the developer carrying member is larger than the
thickness of the developer layer in the developing zone. Across the
clearance, an alternating electric field is formed to transfer the
non-magnetic particles from the developer layer on the developer carrying
member to the latent image bearing member while retaining the magnetic
particles on the developer carrying member surface, irrespective of
whether it is the image area or the non-image area. And, the excessive
non-magnetic particles are transferred back to the developer carrying
member. These steps are alternatingly repeated By doing so, the following
advantageous effects result in combination.
(1) The resultant developed image is of sufficient image density even for a
solid black image, without edge effect, which could not be obtained by
conventional non-contact type developing method.
(2) The image quality is sharp without blur
(3) The magnetic particles are subtantially untransferred to the latent
image bearing member, so that the developed part does not contain the
magnetic particles mixing into the non-magnetic particles This makes a
clear and bright color development possible.
(4) A wasteful consumption of the magnetic particles can be avoided.
(5) Since the magnetic brush is not used in the developing zone, and since
the developing zone is located between the magnetic poles, the area where
the thickness of the developer layer is uniform can be used for
development, so that a uniform effect of the opposing electrode
(developing electrode) can be provided, whereby uniform image quality of
the development is made possible.
While the invention has teen described with reference to the structures
disclosed herein, it is not confined to the details set forth and this
application is intended to cover such modifications or changes as may come
within the purposes of the improvements or the scope of the following
claims.
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